1. Field of the Invention
This invention generally relates to an adsorption system and process and, more particularly, to a pressure swing adsorption system and process of the type wherein a feed gas, e.g. air, composed of oxygen and nitrogen, is loaded in a molecular sieve coke adsorber operative for selectively adsorbing oxygen, and wherein a nitrogen-enriched effluent gas is conveyed from the adsorber, and, yet more particularly, to a method of and arrangement for increasing the enrichment of the nitrogen content of the effluent gas.
2. Description of the Prior Art
Many processes for recovering oxygen and nitrogen from a feed gas such as air are known in the art. For example, for recovering oxygen, one of the chief commercial techniques still in use today is the fractional distillation of liquid air. However, the distillation process is very expensive and requires a major expenditure of capital equipment, and is economically viable only when large commercial plants are built and operated continuously. Small businesses typically cannot afford to build their own fractional distillation plants and, thus, buy liquid oxygen and/or nitrogen directly from the distillation plant.
It also has been proposed to recover both oxygen- and nitrogen-enriched gases utilizing an adsorption process. One type of adsorption process employs silicates, e.g. zeolites, which are effective preferably for adsorbing nitrogen from gas mixtures containing oxygen and nitrogen, so that by conducting a feed gas such as air through a zeolite-filled adsorber, the effluent gas that initially issues from the adsorber effectively is enriched as regards its oxygen content. However, it requires a considerable capital expenditure and considerable energy to regenerate zeolite and, in addition, zeolites are effective only when used with dry air due to their hydrophilic properties. Such air dryers represent still another added expense.
Another type of adsorption process employs carbon-containing molecular sieves of the kind described, for example, in U.S. Pat. Nos. 3,801,513; 3,960,522; 3,960,769 and 3,979,330. Such molecular sieve coke adsorbers have an ultra-fine pore structure wherein the effective average size of the individual pores is less than about 0.3 millimicron. Oxygen gas molecules are smaller, and nitrogen gas molecules are larger, than about 0.3 millimicron. Hence, such molecular sieve coke adsorbers are operative during an initial loading or adsorption phase wherein air is charged to the adsorber selectively to adsorb the oxygen and to discharge a nitrogen-enriched effluent gas therefrom. If the nitrogen-enriched effluent gas is the desired product, i.e. for users requiring nitrogen, then this product gas is collected until its continuously rising oxygen content, i.e. the "impurity", has reached a predetermined limit value. This limit value depends on the purity or concentration of the nitrogen content of the product gas required by a particular user.
Thereupon, the charged molecular sieve coke adsorber is desorbed of the oxygen-enriched residual gas during a subsequent unloading or desorption phase. If the oxygen-enriched residual gas is the desired product, i.e. for users requiring oxygen, then this oxygen-enriched product gas is collected. The alternating adsorption and desorption phases of the abovedescribed adsorption process can be repeated and performed at various pressure changes, or temperature changes, or with one or more vessels in which the molecular sieve coke is contained, to achieve the desired product gas at the desired purity or concentration for either oxygen or nitrogen. For even greater purity, the gas discharged from one vessel can serve as the feed stock for another adsorption vessel. Examples of such processes are described, for instance, in U.S. Pat. Nos. 4,011,065; 4,015,956 and 4,264,339.
As described in the above-identified patents, there were thus obtained very high concentrations of nitrogen in the nitrogen-enriched effluent gas discharged during the loading phase of the pressure swing adsorption system. By way of nonlimiting example, in one particularly advantageous commercial nitrogen-generating system using two alternatingly charged molecular sieve coke adsorbers, the concentration of nitrogen in the nitrogen-enriched effluent gas during the loading phase of each adsorber was on the order of 97.0% through 99.9% nitrogen, depending on such factors as the flow rate of the effluent gas out of the respective adsorber, and the dwell time of the gases within the respective adsorber. The slower the flow rate and the higher the dwell time, the higher the concentration of nitrogen in the effluent gas. However, unless the flow rates were brought down to unacceptably slow, and therefore commercially undesirable, levels on the order of less than about 750 standard cubic feet per hour, and unless the dwell times similarly were brought down to unacceptably long, and therefore also commercially undesirable, levels on the order of over 120 seconds, then the nitrogen concentration thus achieved typically was not higher than 99.9% nitrogen which, although very desirable for the majority of applications, still was not suitable for some applications which required even higher nitrogen purities.
In the semiconductor chip manufacturing industry, an inert nitrogen atmosphere is required for heat-treating the chips in a furnace, and typically the desired nitrogen atmosphere is on the order of 99.999% nitrogen, or, in other words, the total oxygen content per unit volume of the effluent gas is about 5 parts per million. To obtain such high nitrogen purity levels is difficult in commercial distillation plants, and is, of course, expensive.
As for the molecular sieve coke adsorbers which, by contrast to commercial distillation plants, are small-scale systems, such purity levels in excess of 99.9% nitrogen are not readily commercially available, although it has been proposed to inject hydrogen gas into the 99.9% nitrogen content of an effluent gas prior to passing the resulting mixture through a heated palladium catalyst, so that the residual oxygen (on the order of 0.1% oxygen) in the effluent gas would react with the injected hydrogen and produce water which subsequently is to be removed downstream of the catalyst. Although generally satisfactory for its intended purpose of producing product gases having a nitrogen concentration in excess of 99.9% nitrogen in the effluent gas, the above-described catalyst-assisted process has not proven to be altogether convenient in use or commercially viable, particularly in small-scale systems. For example, one drawback with the catalyst-assisted process is that the hydrogen to be injected upstream of the catalyst is not readily available, and would have to be separately purchased since it is not a by-product of the pressure swing adsorption process. The palladium catalyst and the subsystem for heating the same represents still another system complexity and expense. As for the water that is produced, it must be gotten rid of, typically by employing an air dryer such as zeolite. The dryer, of course, represents still another system complexity and expense and, to aggravate matters, the zeolite periodically must be regenerated. The energy consumption required to generate nitrogen-enriched effluent gases with nitrogen concentrations in excess of 99.9% is high and, even for the catalyst-assisted process, represents still another expenditure of energy which it is desired to minimize.